In accordance with the rapid development of mobile communication systems, a compact, low-height, high performance surface acoustic wave device which is one of the key devices constituting the communication equipment, is highly desired.
Although the wire-bonding method has been widely employed in assembling surface acoustic wave devices, the miniaturization of surface acoustic wave devices is limited because of the requirement for a large wire bonding land area. In addition to this, a short-circuit problem has often occurred which is caused by the introduction of conductive foreign objects such as solder particles on the unprotected comb electrode. Therefore, a new experimental face-down assembly method has been reported in Proceedings of the Japan IEMT Symposium (1993), pp. 109-112.
A structure of a conventional surface acoustic wave device assembled by means of said face-down method is explained by referring to FIG. 7 showing a cross-section of a conventional surface acoustic wave device. In FIG. 7, 201 is a substrate, 202 is a comb-electrode, 203 is an electrode pad, 204 is a conductive bump, 205 is a conductive adhesive, 206 is an insulating adhesive, 207 is a package, 208 is an electrode pattern, 209 is as external electrode, 210 is sealing, and 211 is a cover.
As shown in FIG. 7 for the conventional surface acoustic wave device, conductive bump 204 is disposed on electrode pad 203 which is disposed on substrate 201, conductive adhesive 205 is transfer-coated on conductive bump 204, and the device is assembled on electrode pattern 208 disposed on package 207 made of alumina, alumina-glass ceramics, or other materials by means of the face-down method in order to establish an external electrical connection.
In the structure of this device, since the adhesive strength between substrate 201 and package 207 is established only by conductive bump 204 and conductive adhesive 205, this is considered inadequate, and it is reinforced by providing an insulating adhesive 206.
However, in order to avoid any adverse effects on said surface-wave device, a method of applying a high-viscosity insulating adhesive 206 except in the peripheral region of said comb-electrode 202 has been employed in order to avoid the blocking of surface-wave propagation at the area around the comb-electrode 202.
However, although the assembling of the acoustic surface wave device by said face-down method is definitely advantageous for miniaturization of the device, since no wire bonding lands had to be provided, the exact control of the application of the insulating adhesive has been a difficult problem since there are chances of intrusion of insulating adhesive employed to reinforce the adhesive strength of the comb-electrode of the surface acoustic wave device. That is, since the assembly of a surface acoustic wave device has to be carried out under a strictly controlled environment to protect the comb-electrode against the intrusion of outside foreign objects, the application of conventional assembling technology has been difficult and thus impractical.
Thus, another conventional face-down bonding method used to assemble acoustic surface wave devices has been reported in Proceedings of the 1994 Ultrasonic Symposium (1994), pp. 159-162. That means that, without using the reinforcement of an insulation adhesive, the conductive bumps and the electrode disposed on the package are directly bonded by means of heat and ultrasonic energy thus holding the surface acoustic wave device only by means of conductive bumps. Since this is a method without the use of an insulating adhesive, the problems, including the intrusion of adhesive into said comb-electrode, can be eliminated.
However, another possible problem for the surface acoustic wave device is caused by the difference in the thermal stress produced by the thermal expansion coefficient between the surface acoustic wave device and the package. Therefore, the practical application of these face-down bonding methods having the advantage of achieving compact and low height packaging has been considered very difficult.